1. Field of Invention
This invention relates to acquisition of fingerprints and other relieved-surface images via optical means. This invention relates more particularly to the use of total internal reflection and spectral shift of the transmission band in Interference filters due to change in the angle of incidence.
2. Prior Art
Optical fingerprint sensors are usually based on total internal reflection of light. The principle of total internal reflection can be applied in two different ways to create a sensor. The first kind is an absorptive sensor and the second one is scattering-type.
2.1. Absorptive Sensor
The finger is placed on the surface of a glass prism and light is incident through another face of the prism. The angle of incidence is greater than the critical angle and hence all of the light is totally internally reflected from the valleys of the finger. The ridges, however, absorb most of the light. This way the valleys appear bright and the ridges appear dark. This results in a high contrast fingerprint image. Such a sensor has been described by Caulfield et. al. in their U.S. Pat. No. 3,716,301. This sensor suffered from trapezoidal distortion which was later corrected by Bahuguna et. al. in their U.S. Pat. Nos. 5,629,764 and 5,892,599 by holographic means. FIG. 1 illustrates the geometry of the sensor by Caulfield et. al.
2.2. Scattering-Type of Sensor
Again, like in the absorptive sensor the finger is placed on the glass surface of a prism. The incident light, however, is almost normal to the finger. The viewing angle, in this case, is greater than the critical angle. Light from the ridges is scattered in all directions and is received by the optical system whose optical axis is inclined at an angle greater than the critical angle. The light from the valleys, in glass, is confined to a cone whose semi-angle is equal to the critical angle and hence is not received by the camera. This results in a high contrast image of the fingerprint; the valleys appearing dark in this case and the ridges bright. U.S. Pat. No. 6,324,020 by Teng et. al describes such a sensor with reduced trapezoidal distortion and is shown in FIG. 2. U.S. Pat. No. 5,650,842 by Maase et. al also describes such a sensor using a telecentric camera system.